Magnetic recording disks comprising a rigid base which usually consists of an aluminum alloy are chiefly used for data storage. Magnetic storage disks whose recording layer essentially consists of a magnetic material finely dispersed in a binder matrix have long been known and are predominantly used. The steady increase in the storage density of magnetic disks necessitates a corresponding reduction in the thickness of the recording layer and a simultaneous increase in the coercive force. It is becoming increasingly impossible to meet these requirements with dispersion layers. Since it is technically very difficult to compact homogeneous dispersion layers having a uniform thickness of less than 0.5 .mu.m, there is a need for novel coating technologies and novel magnetic materials which make it possible to produce very thin magnetic layers having a homogeneous microstructure and a very smooth surface. Chemical and electrochemical deposition as well as a vapor deposition and sputtering of suitable metals or alloys are possible methods for this purpose. The magnetic properties of thin layers are generally determined by the composition, microstructure and layer thickness. Surface roughness and internal stresses in the film are also important.
Chemically deposited coherent Co-P magnetic layers are now used in commercial thin-film magnetic disks. These layers make it possible to obtain a very high coercive force of from 40 to 80 kA/m and a high relative remanence of more than 0.8 in the plane of the layer at a layer thickness of less than 100 nm. They consist of hexagonally arranged (Co-P) crystallites in an amorphous (Co-P) matrix.
To produce these thin-film magnetic disks, the aluminum substrates are coated in a first step, by a chemical method, with a hard nonmagnetic amorphous (Ni-P) lower layer which is about 20 .mu.m thick and has a P content of from 15 to 20 atom %, making it possible to eliminate small substrate defects. The great hardness of the lower layer improves the tribological properties of the magnetic disk in relation to the head and, in a subsequent surface treatment step, makes it possible to obtain a defined surface roughness which is necessary for reproducible flight behavior of the head. Furthermore, the (Ni-P) surface has a sufficiently high catalytic activity for the subsequent coating with (Co-P) by immersion in a metastable, chemical bath, so that additional surface nucleation with foreign elements which promote deposition, for example Pd, can be dispensed with. Moreover, the amorphous Ni-P structure which is free of particle boundaries favors a homogeneous microstructure of the growing thin (Co-P) layer.
Thus, the surface quality of the nickel-plated aluminum substrates has a substantial effect on the usefulness of the magnetic layers. The surface has to meet high requirements in respect of cleanness and a uniform and defined roughness profile, in order to ensure satisfactory chemical deposition of the magnetic metal layer on the substrate and the required mechanical properties of the corresponding disks possessing metal layers. At present, surface treatmnet is carried out by a method in which, in a first operation, the nickel-plated aluminum substrates are polished in order to eliminate local irregularities, such as peaks and holes. The substrates are then cleaned and dried in order to remove the residues of polishing agent which interfere with the subsequent texturing process. In order to achieve a defined roughness, the clean substrates are then textured in a known manner in one direction which essentially corresponds to the recording direction for information storage, ie. circularly (DE-A 36 01 848). The substrates then have to be cleaned and dried again before the isolated peaks formed during texturing are finally removed manually using a fine abrasive paper. After a final cleaning step, the nickel-plated substrates can be provided with the magnetic layer.
It is an object of the present invention to improve the surface treatment of disk-shaped nickel-plated aluminum substrates suitable for the production of thin-film magnetic disks so that both the number of treatment steps required is reduced and the result of the treatment is improved and hence the yield of thin-film magnetic disks increased.